GLASS. 481 



35 parts of carbonate of soda and 180 parts of broken glass. 

 These materials are first fritted, or heated so as to cause the 

 expulsion of water and carbonic acid, and to produce an agglu- 

 tination of their particles, and afterwards completely fused in a 

 large clay crucible of a peculiar construction. For the first 

 formation of the glass a higher temperature is required, than 

 that at which it is most thick and viscid, and in the proper con- 

 dition for working it. At the latter temperature, the substance 

 possesses an extraordinary degree of ductility, and may be 

 drawn out into threads so fine as to be scarcely visible to the eye. 

 A portion of the plastic mass, on the extremity of a blow-pipe, 

 may be expanded into a globular flask, and prest or bent 

 into vessels of any form, which may be pared and fashioned by 

 the scissors. At a lower temperature, glass vessels become rigid, 

 and when cold brittle in the extreme, unless they be annealed, 

 that is, kept for several hours at a temperature progressively 

 lowered from the highest degree which the glass can bear 

 without softening, to the temperature of the atmosphere. The 

 well-known glass tears, or Prince Rupert's drops, as they are 

 also called, which are made by allowing drops of melted glass 

 to fall into water, illustrate the peculiar properties of unan- 

 nealed glass. The surface becoming solid by the sudden cooling, 

 while the interior is still at a high temperature and conse- 

 quently dilated, the drop is of greater volume than it would 

 be if cooled slowly and equally throughout its mass. Its par- 

 ticles are thus in a state of extreme tension, and an injury to 

 any part causes the whole mass to fly to pieces. The fracture 

 of unannealed vessels, which is the immediate consequence of 

 scratching their surface, has been compared to the effect upon 

 a sheet of cloth forcibly stretched, of injuring its edge in the 

 smallest degree by a knife or scissors. It then ceases to pre- 

 serve its integrity by resisting the tension, and is torn across. The 

 relative proportions of the ingredients of this and other species 

 of glass is subject to some variation. But the oxygen in the 

 bases of window-glass is to the oxgen of the silica, nearly as 

 1 to 4. This glass has a green tint, which is very obvious in 

 a considerable mass of it, occasioned in part, it may be, by the 

 impurities of the materials, but a certain degree of which ap- 

 pears to be essential to a soda-glass. For in all the colourless 

 and finer varieties of glass, it is necessary to use potash. 



Silicates of potash and lime. Plate-glass used for mirrors, 



j i 2 



